Select the true statement regarding the establishment of successful communications. a. transmission power must only be sufficient to overcome signal attenuation since noise cannot be eliminated b. it is critical to know the signal-to-noise ratio (SNR) at the receiver c. when determining the capacity of a communications link it is not important to understand the link characteristics of the medium (guided or unguided) in which the signal will propagate d. the capacity of a transmission link is not dependent upon signal frequency bandwidth